1. Field of the Invention
The present invention relates to a device for measuring displacement and deflection of a main spindle in a machine tool.
2. Description of Related Art
Generally, in the machine tool, a work is supported on an upper face of a bed through a work supporting table and a jig. On the bed, moreover, there is mounted a main spindle, which is moved by a numerical control in the three X-, Y- and Z-axis directions, and on which a tool holder equipped with a tool is mounted, so that the work is machined by the tool. In the related art, for movement of the main spindle in the three-axis directions, there is adopted a moving mechanism, which is mainly composed of a ball screw to be rotated by a servo mechanism, and a ball screw nut. Of this moving mechanism, when the moving mechanism for moving the main spindle back and forth in the axial direction is thermally expanded, an error occurs in the actual moving position of the main spindle. If the work is machined with this error, the machining precision of the work becomes deteriorated. Thus, there has been proposed a thermal displacement correcting device for the ball screw, as disclosed in Japanese Patent Unexamined Publication JP-A-2001-138178. This correcting device has a detector arranged at a position spaced by a predetermined distance in the axial direction from an end face of the free end of the ball screw. The change in the length of the ball screw is measured by the detector, so that the feed of the main spindle is corrected on the basis of the change.
In a tapered hole formed in the spindle of the main spindle, there is inserted and fixed a tapered shank portion of a tool holder that holds a tool. When chips are bitten on the contracting interface between the inner circumference of the tapered hole and the outer circumference of the tapered shank portion, the tool holder and the tool are inclined so that the work is not properly machined. Thus, there has been proposed a machine tool for discriminating the propriety of the rotational deflection of the tool holder. In Japanese Patent Unexamined Publication JP-A-2005-313239, there is disclosed a machine tool, which is equipped in the housing of the main spindle with a distance sensor opposed to the outer circumference of the outer circumference of the tool holder. This distance sensor measures the distance between the outer circumference of the rotating tool holder and the sensor, so that the propriety of the rotational deflection of the tool holder is judged according to the change in the measured distance.
However, the ball screw thermal displacement correcting device, as disclosed in JP-A-2001-138178, has a structure, in which the change in the length of the ball screw at a position spaced from the main spindle is measured, so that it cannot measure the thermal displacement of the main spindle, directly at a portion close to a portion machined. This raises a problem that the measuring precision cannot be improved. Moreover, when a foreign substance such as chips sticks to the end face of the ball screw, the measurement precision becomes deteriorated. This device of the related art has another problem that the thermal displacement of the tool holder or the tool mounted on the main spindle cannot be measured.
On the other hand, the machine tool, as disclosed in JP-A-2005-313239, for judging the propriety of the rotational deflection of the tool holder uses the non-contact type distance sensor. If the foreign substance such as chips sticks to the outer circumference of the tool holder, there arises a problem that the decision precision drops. This device of the related art has another problem that the thermal displacement of the main spindle in the feeding direction cannot be measured. Still another problem is that the sensor has to be cleared of the foreign substance such as chips, if any.